class Solution {//leetcode130——被围绕的区域
private:
    int dx[4]={0,0,1,-1};
    int dy[4]={1,-1,0,0};
    void dfs(vector<vector<char>>& board,int i,int j){
        board[i][j]='.';
        for(int k=0;k<4;k++){
            int y=i+dy[k];
            int x=j+dx[k];
            if(x>=0&&x<n&&y>=0&&y<m&&board[y][x]=='O'){
                dfs(board,y,x);
            }
        }
    }
public:
    void solve(vector<vector<char>>& board) {
        m=board.size(),n=board[0].size();
        for(int i=0;i<m;i++) {
            if(board[i][0]=='O') dfs(board,i,0);
            if(board[i][n-1]=='O') dfs(board,i,n-1);
        }
        for(int i=0;i<n;i++){
            if(board[0][i]=='O') dfs(board,0,i);
            if(board[m-1][i]=='O') dfs(board,m-1,i);
        }

        for(int i=0;i<m;i++)
            for(int j=0;j<n;j++){
                if(board[i][j]=='.') board[i][j]='O';
                else if(board[i][j]=='O') board[i][j]='X';
            } 
    }
private:
    int m;
    int n;
};